This invention relates to a novel method for coating particulate material upon a viewing-window surface of a cathode-ray tube. The novel method is particularly applied to coating a layer of phosphor particles upon the inner surface of the viewing windows of a series of cathode-ray tubes; for example, color-television picture tubes.
In one method of making a phosphor screen for a color-television picture tube, a slurry is prepared which includes phosphor particles, a binder such as polyvinyl alcohol, a photosensitizer for the binder such as ammonium dichromate, an organic filter resin, and a liquid vehicle such as water. In a previous factory process, a series of viewin windows is passed through a semiautomatic machine. A puddle of phosphor slurry is dispensed from a storage container onto the central portion of the inner surface of each slowly-rotating viewing window, which window is part of the faceplate panel of a cathode-ray tube. Each window is rotated and tilted to spread the slurry puddle outwardly to the margins of the window, thereby coating the slurry over the entire window surface. During the spreading step, the coated slurry circulates over the surface and some of the particles therein settle as a layer on the window surface. The excess slurry is then removed, as by rapidly spinning the panel to sling the excess slurry from the panel, recovered, and returned to the storage container, where it is mixed with the slurry therein, and the miture used for coating subsequent window surfaces.
With prior slurry coating factory practice, at the time of dispensing, the slurry has a variable temperature, usually in the range of 26.degree. to 30.degree. C, and a variable viscosity of about 50 to 60 centipoises; and the viewing window has a temperature of about 35.degree. to 40.degree. C. The higher viscosity permits a relatively thick uniform layer of slurry, free of streaks and sags, to be formed on the window surface. As the slurry is spread on the window surface, it is heated to a temperature of about 33.degree. to 38.degree. C with a consequent decrease in viscosity, and phosphor particles settle as a layer upon the surface. The lower viscosity permits the phosphor particles to settle as a uniform layer in the short period of time permitted by the machine cycle. The settled layer that is formed is relatively porous with a screen weight of about 2.50 to 3.60 mg/cm.sup.2.
One expedient for increasing the screen weight without losing adherence of the layer to the surface is to heat the viewing window to higher temperatures, usually in the range of 40.degree. to 50.degree. C at the time of dispensing the slurry. When this is done, the slurry puddle is heated (during spreading) to higher temperatures with still greater decreases in viscosity. The excess slurry that is collected and returned to the storage container, being hotter, raises the temperature of the mixed slurry in the storage container causing its viscosity to drop. There are also corresponding increases in the temperature and decreases in the viscosity of the dispensed slurry. The overall effect of raising the temperature of the viewing window is to disturb the entire system, at times causing catastrophic reductions in the adherence and in the quality of the settled layer.